cgi-bin/code.cgi: new CGI script to update the daily code on a web host

cgi-bin/code.cgi: new CGI script to update the daily code on a web host

This isn't really part of CharmBypass, but it will come in handy on
charm-bypass.com, and where else to put it? It's a CGI script that
allows visitors to update the code. You should probably password
protect it and make it available only to trusted users.

The script is written in POSIX shell script. Why? Because I didn't
really want to venture beyond a single-page, off-line, static HTML
file. But writing a CGI script in POSIX shell was just too dumb to say
no to. It uses sed to parse/edit index.html. Everything about it is
bad.

It can be tested with "python -m http.server --cgi", but the 302
redirect won't work because python doesn't support it.